The embodiment of a ground fault protection is, at the present time, determined primarily by the size of the network, its grounding and the regulations which apply to electric heavy current installation with regard to permissible voltages in the case of ground faults on a grounded component. In view of the different appearances of the networks, therefore, several different types of measurement criteria are currently used.
An ungrounded network may be used when the total length of line of the network is not too great. In that way, any ground fault current which appears will be limited by the capacitive reactance of the network to ground and by a possible transition resistance existing at the site of the fault. In this connection, directional current relays are used which are sensitive to ground fault currents that are capacitive relative to the neutral point voltage.
Direct-grounded networks are seldom used at the distribution level since any ground fault current could assume very high values. On the other hand, direct grounding is commonly used for the transmission networks. Since direct grounding implies that the neutral point voltage of the network is zero, the ground fault current is utilized only for selective ground fault detection or for disconnection of a faulty transmission line.
Grounding via neutral point resistors larger than 5.OMEGA. occurs in small and medium-sized networks. The neutral point resistor is chosen such that a sufficiently active current (or--as it is also called--resistive current, that is, a current which is in phase with the neutral point voltage) is obtained in the case of a ground fault. The network is protected by directional current relays which are sensitive to ground fault current and which are supplied with the current which passes via the neutral point resistor.
Grounding by means of a neutral point reactor and a neutral point resistor occurs in large networks, in which the capacitive ground fault current would otherwise become too high. The capacitive ground fault current is compensated for by the neutral point reaction so as to obtain a tuned network. For selective disconnection of a faulty component, current direction relays are used which are sensitive to resistive ground fault current, that is, the current through the neutral point resistor. In some applications a special automatic system is available which takes care of the disconnection and connection of the neutral point resistor so as to provide a chance of self-extinguishing of the fault before a relay starts operating and disconnects the line.
Grounding by means of a neutral point reactor occurs in large distribution networks. Otherwise, the same conditions apply as in the case of grounding by means of a reactor or a resistor; however, it is presupposed that the internal resistance of the neutral point reactor will be sufficiently high as to enable the evaluation of a resistive current component.
The components and systems for ground fault measurement, identification and disconnection which are used at present cannot, as is partly clear from the above description, be made identical because of different grounding principles. Nor is it possible to achieve the desirable sensitivity and speed.
In direct-grounded networks, however, it is theoretically possible to achieve a relatively high sensitivity of the ground fault protection devices with a possibility of selective detection of faults with high transition resistances. This type of grounding primarily applies to the transmission networks.
Ungrounded networks also provide relatively good possibilities of selective detection of ground faults with a high transition resistance. However, ungrounded networks are less common, since only small networks would normally be operated in this manner. In addition, ungrounded networks are normally not used in view of the risk of intermittent ground faults.
In networks which are resistance-grounded corresponding to a ground fault current of 2-15 A and networks with combined reactive and resistive grounding, directional current relays which are sensitive to resistive ground fault currents give varying sensitivity to ground faults with a high transition resistance. The sensitivity is largely dependent on the size of the network. Generally, however, it can be said that large networks give limited possibilities of good selective detection of high-ohmic ground faults.
The three most important reasons for the limited sensitivity of current direction relays which are sensitive to resistive ground fault current are as follows:
1. In the case of low ground fault currents, where the capacitive or the inductive component is predominant, the angular fault of the current transformer may cause incorrect measurement.
2. In the case of faults with a low degree of propagation, power and current direction relays have difficulties in measuring I.times. cos .phi. in the case of large values of .phi., that is, in the range .phi.=80.degree.-90.degree..
3. For practical reasons it is not possible to set the sensitivity of the relays at an arbitrarily high level. The reasons for this may be leakage currents in isolators, the possibility of spontaneous contacts of the line by vegetation or the possibility of saline contamination of lines near sea shores. If the sensitivity of the ground fault protection devices is set too high there is, therefore, a likelihood of unjustified or false-alarm trippings.
It is an express wish on the part of the distributors of electrical power to detect ground faults with a higher transition resistance at the site of the fault than is possible at present (.about.3 k.OMEGA.). In many transmission networks ground faults with high fault resistances may also occur. These faults are difficult to detect with, for example, impedance relays.
It is a well known fact that ground faults which are not located and disconnected in time may lead to personal injury or fire. Of special interest in this connection are reversed ground faults, that it, faults in which a phase is interrupted and a ground fault involving feeding via the load object arises in the phase after the moment of interruption. With presently available detection equipment these faults may remain undiscovered for a long period of time.
In addition to the above general description of the various designs of ground fault protection devices and the associated problems, the description of the prior art should also include a reference to Berggren U.S. Pat. No. 4,529,929 assigned to the assignee of this application.